In a conventional apparatus for detecting the ambient humidity, hairs or specific chemical fibers are utilized for measuring the ambient humidity by using the expansion and contraction characteristics of these materials. However, the accuracy of the measured humidity by means of such materials is very low, and therefore, such humidity measuring means is not practical for precise control of the ambient humidity. Recently, a humidity sensing element the electrostatic capacitance of which varies in accordance with the ambient humidity, has been developed. When such a sensing element is utilized as a transducer, an electrical circuit which measures the capacitance of the sensing element is required to produce an electrical signal indicative of the capacitance and therefore the humidity. There are several possibilities for such an electrical circuit. For instance, an A.C. bridge or a resonant circuit may be used to measure the capacitance of the sensing element. However, when employing the above mentioned techniques, troublesome steps are required and furthermore, the electrical circuits for achieving the above mentioned techniques are too complex in construction.
Some of the inventors of the present invention had already proposed in a published British patent specification Ser. No. 2 017 924 A) a new humidity sensing element the electrostatic capacitance of which varies in accordance with the surrounding relative humidity and thus indicates the relative humidity. In addition, the inventors of the present invention had already prosed in another published British patent specification Ser. No. 2 020 434 A) an electrical circuit for displaying the realtive humidity measured by the sensing element of the above mentioned variable-capacitance type.
The above mentioned humidity sensing element is quick in response, and has a relatively high accuracy, while the element can be manufactured with low cost. When such a sensing element is used, it is preferable to measure the time required for the voltage across the sensing element which is charged by an electric current through a resistor, to rise 0.6 to 0.7 times the voltage of the power source applied to the charging circuit, in order to accurately measure the ambient humidity inasmuch as the equivalent series resistance and the dielectric loss angle of tan .GAMMA. in the humidity sensing element are much greater that those in ordinary capacitive elements.
The inventors of the present invention have devised various improvements as to electrical circuits which measure the capacitance of the above mentioned humidity sensing element the electrostatical capacitance of which varies in accordance with the ambient relative humidity, and thus indicates the sensed humidity. These improvements, which will be described hereinbelow in detail, are significant when it is intended to apply such an electrical circuit to apparatus for controlling humidity or to apparatus which should be controlled in accordance with the sensed relative humidity.
It is, therefore, a primary object of the present invention to provide apparatus for controlling humidity in which the ambient relative humidity is accurately measured.
Another object of the present invention is to provide apparatus for controlling humidity with which the ambient relative humidity may be readily adjusted for a desired value.
A further object of the present invention is to provide apparatus for controlling humidity in which the ambient relaitve humidity is accurately measured irrespectively of the fluctuation in the power source voltage.
A still further object of the present invention is to provide apparatus for controlling humidity, the operation of which is hardly influenced by external noises.
A still further object of the present invention is to provide apparatus for controlling humidity, which is simple in construction and is low in cost.
In accordance with the present invention, there is provided apparatus for controlling humidity, including a humidity sensing circuit for producing an output signal indicative of the ambient relative humidity; and means for controlling the humidity in accordance with said output signal, said humidity sensing circuit comprising: (a) a humidity sensing element, the electrostatic capacitance of which varies in accordance with the ambient relative humidity; (b) a resistor connected in series with said humidity sensing element, said series circuit of said resistor and said humidity sensing element being connected to a D.C. power source so that said humidity sensing element is charged through said resistor; (c) a pulse generator for producing an output pulse train signal; (d) a switching circuit responsive to said pulse train signal for discharging charge prestored in said humidity sensing element; and (e) a voltage comparator responsive to the voltage across said humidity sensing element and to a reference voltage for producing an output signal which indicates that the humidity sensed has a predetermined relationship with respect to a reference humidity.